Heavy-current connections, so-called generator output lines, are the electrical connection between a stator winding of a generator within a generator housing and an associated generator transformer or unit-connected transformer in power plants. This electrical connection usually has a phase-isolated construction, i.e. each of the three current conductors of the three-phase system is arranged in a separate, likewise electrically conductive casing, a so-called output line cladding (also known as a cladding tube), having a coaxial structure. Generator output lines of this type are disclosed, for example, in DE 25 45 832 C2 and DE 196 19 729 A1.
For better understanding, reference is first made to FIGS. 4 and 5 for describing the construction of a conventional generator output line. The generator output line 10 substantially includes a cylindrical inner conductor 11 and a cylindrical cladding tube connection region 12 which is arranged concentrically with respect to the inner conductor 11 and encapsulates the inner conductor. The three isolated cladding tube connection regions 12 associated with the three inner conductors 11 of the three phases are linked up with the three generator bushings 16 in a connection region in the generator base 14 underneath a generator.
In order to increase the mechanical stability with respect to vibrations, the inner conductors 11 are connected to the generator bushings 16 via flexible expanding copper strips 18. For the same reason, the cladding tube connection region 12 may also have an expanding bellows (not shown). The inner conductor 11 is also sealed off from the inner wall of the cladding tube connection region 12 by a disk bushing 20, for example made of cast resin. In conventional generator output lines 10, both the cylindrical inner conductors 11 and the cladding tube connection region 12 are made of pure aluminum Al99.5. Connected to the generator output lines 10 are conductor tubes 22 and cladding tubes 23, each having a larger diameter than the inner conductors 11 and the cladding tube connection region 12, respectively.
The electrical voltage level of the generators and the generator output lines is nowadays between 12 and 36 kV (so-called medium-voltage level), depending on the rated power of the generator. According to this voltage level, the clearance between the inner conductor and the cladding tube (ground potential) has to be at least 120 to 320 mm. For example, in the case of steam power plants having rated generator powers of 600 to 900 MW and a generator voltage of 21 to 27 kV, a current intensity of between 18 and 25 kA is produced both in the inner conductor and in the outer cladding tube.
These currents result in I2R losses which lead to an increase in the temperature of the inner conductor and of the cladding tube. In order to be able to keep to the maximum permissible temperatures, as set by the IEC and the ANSI, of 90° C. for the inner conductor and 70° C. for the cladding tube, the diameters and the wall thicknesses of both the inner conductor and the cladding tube need to be dimensioned accordingly.
Normally, there is sufficient space to install the three single-phase generator output lines when they are made of aluminum, irrespective of the voltage and current values. Only in the case of a connection region in a generator base, as shown in FIG. 4, are copper inner conductors conventionally used owing to the technical/electrical requirements and the very small amount of space available as well as the small gap between the generator bushings on the generator. By virtue of their better electrical conductivity, these copper tubes require smaller diameters and smaller wall thicknesses than aluminum tubes to carry the same current intensities whilst keeping to the permissible temperatures.
In addition to the increased material costs, the increased problems presented when welding and the increased weight of copper in comparison with aluminum, a particular problem with copper is the fact that it is necessary to provide a special soldered or welded electrical connection 24 between the thinner copper inner conductor 11 and the aluminum conductor tube 22 connected to it, which has a larger diameter.